1. Field of the Invention
This invention relates to a method for producing acrylic acid comprising introducing propylene as a raw material in high concentration into a reactor which has a first reaction zone and a second reaction zone formed of different reaction tubes, and improving water concentration in an acrylic acid-containing solution in an acrylic acid absorption column to a level in the range of 1–45 wt. %, thereby preventing the solution from polymerization at the subsequent steps in the process of the production.
2. Description of the Related Art
Acrylic acid is used in coating material, fiber processing, leather processing, and building material besides being used for acrylic fiber copolymer or as an emulsion for adhesive agents. The demand for this compound is now increasing. Thus, it is generally produced by the reaction of catalytic gas phase oxidation of propylene, for example, with a view to realizing the principle of mass production using an inexpensive raw material. The raw material gas is generally supplied at a concentration in the range of about 4–7 vol. % because the reaction of catalytic gas phase oxidation has the possibility of belonging to the range of explosion, depending on the proportion of molecular oxygen to be used in the reaction of oxidation. Since the raw material gas is preferred to be used at a high concentration for the purpose of improving the efficiency of production, various modifications have been proposed for the supply of the raw material gas at a high concentration.
The official gazette of JP-B-62-17578, for example, discloses a method for producing acrylic acid by using a molybdenum-bismuth type catalyst in the reaction of two-stage catalytic gas phase oxidation performed by subjecting a mixed gas composed of propylene, air, and steam to a first-stage reaction for converting propylene into acrolein and subjecting the gas formed by the first-stage reaction to a second-stage reaction for converting acrolein into acrylic acid, endowing the mixed gas supplied to the first-stage reactor with a composition such that the molar ratio of molecular oxygen to propylene falls in the range of 1.1–2.0:1, the propylene concentration in the range of 7–13%, and the steam concentration in the range of 2–30%, controlling the temperature of the mixed gas prior to the introduction into the catalyst bed to a level of not higher than 260° C. and adjusting the reaction conditions on the first-stage catalyst bed so that the reaction temperature falls in the range of 260–370° C. and the duration of contact in the range of 1–8 seconds, producing the mixed gas between the first-stage reaction gas supplied to the catalyst bed in the second-stage reaction and the gas resulting from adding air or oxygen gas to the waste gas separated from the gas formed by the second-stage reaction, allowing the oxygen content to assume such an amount that the molar ratio of the total (X) of the amount of oxygen in the mixed gas subjected to the first-stage reaction and the amount of oxygen added herein to the amount of propylene (Y) in the mixed gas subjected to the first-stage reaction, (X:Y), falls in the range of 1.6–2.8:1, and controlling the temperature of the mixed gas prior to the introduction into the second-stage catalyst bed to a level of not higher than 280° C. This method specifies such factors as the kind of catalyst, catalyst temperature, and propylene:oxygen ratio because an undue increase of the propylene concentration results in preventing oxygen from being supplied in a fully sufficient amount owing to the restriction imposed by the explosion limit and lowering the yield of reaction as well and for the sake of solving the problem such as the runaway reaction of combustion due to the autoxidation of acrolein in the second-stage reaction.
The official gazette of JP-B-62-17,579 discloses a method for producing acrylic acid by a reaction of two-stage catalytic gas phase oxidation performed by subjecting a mixed gas comprising propylene, molecular oxygen, and steam to a first-stage reaction thereby converting propylene into acrolein and subjecting the gas formed by the first-stage reaction to a second-stage reaction thereby converting acrolein into acrylic acid, causing the reaction of first-stage catalytic gas phase oxidation to proceed in a specific reaction zone having the molar ratio of molecular oxygen/propylene in the range of 1.17–1.66, the ratio of steam/propylene at a level of not more than 4, the propylene concentration in the range of 7–15%, and the gas pressure in the range of 0.4–1.5 kg/cm2 (gauge pressure), suddenly cooling the gas formed by the first-stage reaction to below 280° C., and adjusting the gas so that the ratio of molecular oxygen/propylene falls in the range of 1.75–2.5, the ratio of steam/propylene falls in the range of 1–5, and the gas temperature falls below 280° C.
The official gazette of JP-A-2001-64,227 discloses a method for forming a gas produced by a mixed gas containing acrylic acid by supplying a reaction composition containing more than 7 vol. % of propylene, oxygen, steam, and the balance including an inert gas to a first reactor furnished with a reaction tube packed with a catalyst thereby forming a mixed gas containing acrolein, cooling the gas with a heat exchanger, and then supplying the acrolein-containing gas to a second reactor thereby obtaining a mixed gas containing acrylic acid. Since an undue increase of the amount of propylene has the possibility of suffering combustion and reaction to proceed explosively, the practice of controlling the amount of propylene in the approximate range of 4–7 vol. % has been in vogue to date. This invention contemplates rigidly controlling the temperature of the gas entering the first reactor and the second reactor by cooling and supplying to the gas on the verge of being introduced into the second reactor with a stoichiometrically sufficient amount of oxygen thereby permitting the method to use propylene of high concentration. According to the disclosure, the acrylic acid content in the aqueous acrylic acid-containing solution derived from an absorbing column is in the range of 32–55 wt. %.
The official gazette of JP-A-2002-161,066 discloses a method for oxidizing propylene into acrolein in a first reactor and continuously producing acrylic acid from the acrolein in the second reactor, which method produces the acrylic acid by the reaction of gas phase oxidation of propylene without entailing the deposition of a solid matter as to a gas cooler furnished behind the outlet of the first reactor even after a protracted continuous operation. This method which produces acrylic acid while having the exhaust gas (quencher gas) emanating from an acrylic acid absorption column circulated to the reaction of catalytic gas phase oxidation for converting propylene into acrolein is characterized by adjusting the propylene concentration to a level in the range of 5–15 mol % and the molar ratio of the steam and oxygen gas to propylene in a specific range and allowing the acrolein to be converted into acrylic acid by the reaction of catalytic gas phase oxidation after the acrolein-containing gas has been cooled. By this method, it is made possible to effect continuous production of acrylic acid over a long period while preventing the accumulation of a solid matter in the reaction system and the rise of pressure in the first reactor, the second reactor, and the gas cooler even when the quencher gas is recycled to the first reactor.
Acrylic acid, however, is an easily polymerizing compound. When the raw material gas of high concentration is used with a view to heightening the productivity of the method, this method is liable to generate an acrylic acid polymer at the step for absorbing acrylic acid and the subsequent step for purification the absorbed acrylic acid other than the reactor. Since various columns for purification continue production of acrylic acid while preventing occurrence of polymerization by adjusting such factors as the pressure of distillation, temperature, and amount of feed, the control of these factors is not easy because changes in these conditions affect other conditions. When the concentration of acrylic acid is depressed by a variation in the composition of the bottom liquid of an absorption column, for example, the maintenance of distillation conditions in a high acrylic acid concentration entails generation of an acrylic acid polymer in a distilling column. Though it is not to the extent of requiring forced suspension of the operation due to blockage, but possibly results in degrading the quality of acrylic acid produced due to the inclusion of the polymer and consequently depressing the yield of the product.
The purification step of acrylic acid is accomplished more often than not by adopting distilling columns called a dehydrating column, a light ends cut column, and a heavy ends cut column, concatenating the plurality of distilling columns endowed with different functions, and operating them jointly thereby performing a series of purification. Thus, the control of this purification step is rendered further difficult by the fact that a variation in the composition of the bottom liquid of any one of such distilling columns necessitates a due change in the purification conditions of the subsequent steps.
The solution, the exhaust gas, and the like which are discharged from the step for producing acrylic acid at times contain the raw material compound, the product, and other useful compounds. The cyclic use of such discharged substances in the process of production, therefore, can improve the yield of production. The composition of the exhaust gas, for example, is varied as with the distillation conditions and the generation of the polymer proceeds like a chain reaction. In the method for the production of acrylic acid particularly included a step for recycling the exhaust gas, therefore, it is extremely difficult to control the distillation conditions constant.